Cementitious products, such as masonry, paving, paving flags, paviours, flagstones, blocks, bricks, tiles, slates, posts, lintels, sills, slabs and panels and claddings and linings for walls, ceilings, roofs and floors have been made for many years by pressing or moulding processes. Hereinafter such pressing or moulding processes will be generically referred to for convenience as "pressing". Cementitious products are typically made of a cementitious mix of water, cement, sand, aggregate, filler, etc., with or without a colouring pigment. Instead of pigment, the aggregate itself may provide colour to the cementitious product.
A typical pressing process involves the use of what is commonly known as a static block making machine. Pallets made from metal or wood are fed by a conveying means into the block making machine which comprises a mould, a cementitious mix (concrete) filling means, one or more compacting devices, optional vibration devices, a stripping device, and an outgoing conveying means. The pallet, which forms the profile of one side of the block, is located under the mould in the machine. The mould comprises one or more side walls each defining a side of the desired block, and preferably the compacting and stripping device forms the final side of the block. Once the pallet has been located, the mould is lowered on to the pallet and held in frictional contact to define a mould cavity comprising the upper surface of the pallet, and the surfaces of the side walls of the mould.
The mould cavity is then filled with a cementitious mix (concrete) of the desired composition optionally using vibration or other levelling means to achieve a desired profile of concrete. A compacting head, which preferably will also be used for stripping, is then lowered on to the upper surface of the concrete in the mould cavity and the concrete is compacted by the force exerted by the compacting head, such force being direct pressure and/or vibration. Optionally, the mould cavity is then filled with a second cementitious mix (concrete) of another desired composition again optionally using vibration or other levelling means to achieve a desired profile of cementitious mix (concrete). A compacting head, which preferably will also be used for stripping, is then lowered on to the upper surface of the concrete in the mould cavity and the concrete is compacted by the force exerted by the compacting head, such force being direct pressure and/or vibration. The use of two filling operations allows the manufacture of a block which primarily comprises a strong cost effective non visible layer in use and a more pleasantly aesthetic visible layer in use that may contain more expensive ingredients.
The compacting forces are stopped and the mould is then raised from the pallet with both the pallet and the compacting head, now acting as a stripping device, being held in stationary alignment. When the mould has been raised clear of the green concrete block the compacting/stripping head is also raised clear of the green concrete. The formed green block of cementitious mixture (concrete) is then conveyed out of the block making machine on the pallet on which it was formed.
Optionally, the formed green concrete block on its pallet is conveyed into a secondary processing area, such as a washing station, to effect an exposed aggregate finish and then into an area where it is allowed to cure to a hardened state. Having cured to a sufficiently hardened strength the block is then conveyed on the pallet to a depalleting station. The depallated block is then optionally transported to another secondary processing area where it is subjected to the action of secondary processing apparatus such as a shotblasting machine, bush hammer, chain flailer or the like and thence into an area where it is packaged for delivery. In some cases the depalleted blocks are put into yard storage for a period to time before returning to a secondary processing area. The pallet is conveyed back into the block making machine and the making cycle restarts. Many other presses, pressing processes, apparatus and machinery and secondary processes and processing machinery, other than those described, are available. By way of example, hermetic pressing machines, hydraulic pressing machines that require no vibration, hammer action presses, presses using stamper plates, and presses with roller action are all used to make the same product type.
Conventionally, the compacting and stripping device employs a hard pressing shoe, also known as a stripper shoe, made from a metal or other hard or unyielding material, which alone acts on the cementitious mix (concrete) contained within the mould. The effect of the action of the shoe on the cementitious mix in the mould cavity is shown diagrammatically and to an enlarged scale in FIGS. 1 and 2 of the accompanying drawings to which reference will now be made. In FIG. 1, an upper shoe 1 is shown in the position in which it is pressing against an initially compacted charge of cementitious mix 2 in a mould cavity 3 which in turn further compacts the charge of cementitious mix by pressing it against a pallet (not shown). The cementitious mix 2 includes fine particles (sand and cement), so-called "fines" 4, and coarser particles of varying sizes made of a suitable aggregate 5 such as gravel or stone chippings.
As will be apparent from FIG. 1, the action of the unyielding shoe 1 has the effect of forcing the aggregate particles 5 downwards, as indicated by the illustrated arrows 7 and the fines 4 to move upwardly with respect to and around, the coarser aggregate particles 5, as indicated by the arrows 6, so that in the formed green cementitious product 8 shown in FIG. 2, the aggregate particles 5 lie beneath the upper surface 9 which is formed of fines 4 and which presents the exposed surface of the product in use.
Moreover, because the pallet is also made of unyielding material, the lower surface 10 of the cementitious product 8 also ends up being formed of fines 4. Whilst surfaces made of fines are perfectly satisfactory for so-called "commodity" walling blocks whose surfaces may be covered in use, this is certainly not the case with cementitious architectural masonry products such as paving flags and blocks which present at least one exposed surface for view and for wear. If cementitious products having exposed surfaces formed of fines are used, then not only are these surfaces unsightly, but they are more subject to wear which reduces still further their aesthetic appearance.
Accordingly, with cementitious products which in use have at least one exposed surface, it has become desirable to provide the exposed surfaces with surface finishes that are not only aesthetically pleasing but also may be more resistant to wear.